Illegal Southwest border crossings were down 40% last month, according to just released Customs and Border Protection numbers -- a sign that President Donald Trump's hardline rhetoric and policies on immigration may be having a deterrent effect.

Secretary of Homeland Security John Kelly himself announced the month-to-month numbers, statistics that CBP usually quietly posts on its website without fanfare.

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According to CBP data, the 40% drop in illegal Southwest border crossings from January to February is far outside normal seasonal trends. Typically, the January to February change is actually an increase of 10% to 20%.

The drop breaks a nearly 20-year trend, as CBP data going back to 2000 shows an uptick in apprehensions every February.

The number of apprehensions and inadmissible individuals presenting at the border was 18,762 people in February, down from 31,578 in January.

It will still take months to figure out if the decrease in apprehensions is an indication of a lasting Trump effect on immigration patterns. Numbers tend to decrease seasonally in the winter and increase into the spring months.

But the sharp downtick after an uptick at the end of the Obama administration could fit the narrative that it takes tough rhetoric on immigration -- backed up by policy -- to get word-of-mouth warnings to undocumented immigrants making the harrowing journey to the border.

"Firmness pays," said Dan Stein, president of the Federation for American Immigration Reform, an advocacy group that supports vastly restricting immigration to the US.

"This is encouraging news," Kelly said in his statement, citing the increase in apprehensions between October and the end of last year. "However, since President Trump took office on January 20, we have seen a dramatic drop in numbers."

Kelly noted that fewer apprehensions means fewer people making the dangerous journey north to the border.

DHS has also noticed a corresponding increase in the amount that smugglers, called "coyotes," are charging to take people to the border -- essentially the only way to make it through cartel-controlled smuggling routes. In some areas, Kelly said, fees have ranged from $3,500 to $8,000.

"We will remain vigilant to respond to any changes in these trends, as numbers of illegal crossings typically increase between March and May," Kelly said. "However, the early results show that enforcement matters, deterrence matters, and that comprehensive immigration enforcement can make an impact."

While Stein argues that the deterrence only serves to cut down on fraud, however, and legitimate refugees will still make their way to the US, opponents of Trump's policies say his actions mostly harm vulnerable people like women and children that the US system is designed to protect.

"Well, the bullies can gloat and preen that they chased the skinny kids off the block," said Leon Rodriguez, a former Obama administration director of US Citizenship and Immigration Services. "We need to understand what has occurred here. Poor people -- in many cases, mothers with children or children alone, fleeing intolerable violence and poverty -- have been scared away. Many of those are people with legitimate asylum claims that would ultimately have been granted had they actually reached ports of entry."

Factors can be complex

While the numbers make the case that Trump's efforts to restrict illegal immigration are already working to deter would-be migrants, a former CBP official cautions that there are likely a more complex range of factors at play.

Former Obama administration Immigrations and Customs Enforcement chief and DHS counsel John Sandweg, now in private practice, says that changes in the flow of legal and illegal immigration depend on a range of factors, including seasonal and long-term trends, economic situations around the world and the availability of money to pay smugglers.

"It may be premature to point to the rhetoric as the cause of the numbers dropping," Sandweg told CNN. However, "it's true that deterrence is a piece of the puzzle, and if there's a perception that people will be returned, there may be a decrease in flow."

Sandweg also noted that the Obama administration had been working to counter the driving factors of illegal immigration, including working with Mexico to cut off smuggling networks and efforts to improve the economic and violence situations in Central America that many migrants are trying to flee.

The most humane policy would be substantially boosting the number of immigration judges, cutting down on the years-long backlog that is clogging the immigration courts and not contributing to an environment where undocumented immigrants perceive they can build lives in the US without being sent home, he said.

"The problem is more complex, and what I would hate to have is people thinking there's a simplistic solution to the problem," Sandweg said.

Trump has been pushing a host of tough immigration policies even without changing the laws, including tightening the standards for applying for asylum in the US, setting the stage for vastly increased deportations and detentions and giving immigration officers more authority to pick up undocumented immigrants even if they are not a threat to communities. He has also sought to prevent travel from certain high-risk nations and suspend the refugee program to the US temporarily through an executive order that has been held up by the courts.

Supporters of the measures say the cumulative effect of Trump's changes, plus his hardline rhetoric throughout the campaign and statements like Kelly saying the administration would consider separating children from their parents in detention as a deterrence measure combine to send a message to would-be immigrants.

"Trump's tough talk tells you all you need to know," Stein said. "It's the language people understand. It's obviously the language people on the other side of this issue dislike, but ... it's a sad fact but people try to game the system. There's a huge amount of demand to try to live in a country like the United States, so in the end, the only way you can make sure everybody plays the rules and ensure fairness is by making sure no one can jump the line, so tough talk, making sure that anyone, smuggler, alien, that they get it."

What the numbers show

Border apprehensions were down across the board -- with the decline among families and unaccompanied minors even sharper. Family unit apprehensions were down 66% and unaccompanied children were down 55% last month.

It's the first time since at least 2000 that apprehensions at the Southwest border have dropped in February, according to CBP data.

Numbers had also dropped in January, by almost 30% total and roughly 40% among children and families.

But experts hadn't been sure what to make of those numbers, as apprehensions typically drop in January and the decrease was actually smaller than the year before.

Right after the election, officials sent an additional 150 agents to the border to handle an increase in apprehensions.

The influx propelled apprehensions to near five-year highs in October.

In February, apprehensions were near five-year lows.

CNN's Catherine Shoichet and Polo Sandoval contributed to this report.